I took my recently acquired Fiat Idea to my local car mechanic today ....we have been using it for a few weeks and it's great .So we decided to run it in to him to give it the once over full service and a cambelt ...The first thing his son did was connect it up to a reader, like a little fat Ipad ...and got the details from the onboard computer and the ECU ...It all checked out from what I could see looking over his shoulder ..it was all in Greek.... Two things he pointed out to me ...the kilometre reading on the dash says 122 thousand and something but the reader says the car has only done 115 thousand and something ...how does that happen? anyone come across that before ?...and the interesting thing is the RPM reading on the reader tell us that the highest revs it ever run at is 6000 rpm ...so it's never been thrashed .....can that be true ? it's from Italy ..after that we stuck it up on the lift and I was able to have a snoop about It all looks tidy for a 2004 motor no rot at all ..gonna need new pads on the front ..and a bottom engine mount thingy, a set of plugs , oil filter, cambelt and tensioner ,so not so bad ....

Recorded mileage is a common reoccurring on the MutliECUScan forums.

It is also my experience.

The odometer value can only be considered to be valid when read form the instrument cluster on a car where ALL original components are intact.

On my Croma 2005 the Odometer value differed from the ECU value and I had owned that car from new and know 100% for fact that no changes were made or units replaced.

Many more modern vehicles record so called mileage in the instrument cluster odometer, engine ECU, ABS system ECU, etc.

It seems that these recorded values often differ and to my knowledge nobody has come up with a plausible reason.

For example 1 mile = 1.609km (to 3 decimal places). If you use the common 1.6 conversion value the over 1000s of miles/kms then tracking differences can and will occur. So the ABS may use 1.61, the ECU 1.6, the odometer 1.609, etc.

LARGE discrepancies would potentially be a reason to dig deeper.

Oh interesting ...so the data is collected from I guess the transmission by counting revolutions? and recorded on the ECU and the odometer and used by the ABS and they all convert it from revolutions into kilometres I'm in Greece, and the car an Italian import so no need to do a conversion from kilometres to miles, so it must be either a part has been changed or the revolutions to kilometre calculation varies ...?

isthisagoodidea said:

Oh interesting ...I'm in Greece, and the car an Italian import so no need to do a conversion from kilometres to miles, so it must be either a part has been changed or the revolutions to kilometre calculation varies ...?

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The speedometer/odometer drive will come from either the gearbox differential output and/or the ABS wheel sensors. ABS offers redundancy in that 4 sensors are available and resolution (pulses per revolution) is far greater.

Like I said nobody has factually pinned down and explained why the values in different units differ. For example on the Croma 2005 1.9Mjet the engine ECU is not connected to the CAN Bus whereas the body computer, ABS and instrument cluster and service node are. The TRW steering node is primarily not connected to the CAN bus *except* that it gets the vehicle speed from the ABS unit via the CAN bus. So in the case of the Croma 2005 1.9 Mjet the units seems to be cobbled together.

If we focus on a 100% CAN bus system and assume the speed/distance sensor pulse go into the instrument cluster which then converts these to km then how does the odometer value get to the other units? If it a message saying "set 1000", then "set 1001" then "set 1002". If this were the case then dropped or lost messages would not be an issue because if the "set 1001" was lost then the "set 1002" would correct everything. However if the message is "set +1" (or "INCrement +1) then a dropped or lost message will causes differences in the values.

I've never seen speed differences between units. Speed is a critical safety real-time value that would have the highest priority. Pulse to instrument cluster get converted to speed and increment the odometer. Odometer is not a critical safety value so one could *speculate* that sending "msg 123456789" is a waste of bandwidth and could have serious side effects. However a low priority of "msg +1" is sacrificial in the overall system design as the instrument cluster odometer is the de-facto LEGAL homologation requirement. Hence why the real odometer value is the one that matters.

My 500 has a discrepancy as well.

Odometer reads 166799 but a scan of computer says 157... and that something was done to it at 50000.